During camping in cold weather, people often ask why their sleeping bags are not warm enough to comfort them. The answer depends on the R-value of the sleeping pads; if the sleeping pad is under R=5.38, the result varies in getting a colder sleep during the night.
R-Values of sleeping pads
The R-Values of sleeping pads measure the level of the pads' insulation or how it covers heat loss when the temperature gets cold. To keep it short, sleeping places with higher R-values will keep you warmer than those with lower R-values.
In the industry, most sleeping pad manufacturers present their products to independent testing labs to know and test their R-values before the release. The methodology and structure are developed and standardized using the industry standard ASTM F3340-18 which came out in 2020; the bar standard informs the consumers to have the clarity to choose and compare which R-Values of different brands of sleeping pads.
Temperature ratings of Sleeping bags
If sleeping pad manufacturers have their rating standards, the sleeping bag has its field too. Many sleeping bag manufacturers send their products to test their Comfort or Lower limit temperature rating, submitted to the European Norm 13537 standard or the ISO 23537 sleeping bag rating standard.
The testing process to generate temperature ratings requires a sleeping pad with an R-Value of 5.38, assuming the consumers are wearing warm clothes from top to bottom.
Since sleeping bag temperature ratings are based on using a sleeping pad with R-values of 5.38 and wearing warm clothes such as long pajamas and a Santa hat, it is reasonable that your sleeping bag will not keep you warm if the pad is lower than 5.38.
But R-values can be added to stock up. If you consider hiking or camping in a freezing area, consider carrying two sleeping pads instead of one in your cold adventure for additional warmth and comfort.